Abstract: Multi-walled carbon nanotubes(MWCNTs) reinforced Al matrix composites were prepared by friction stir processing(FSP). The microstructure and hardness of the composites were investigated. The results show that the MWCNTS/Al is composed of fine equiaxed grains but the grains are uneven. The interface between MWCNTs and the matrix is well binded. A large numbers of dislocations were distributed in the interface. MWCNTs mainly distributed within the aluminum grains. The grain refinement is realized by recrystallization through a subcrystal nucleating mode. The nucleis form nucleus in the primary grains and the formed subcrystal. Therefore, the grains are refined gradually. With increasing MWCNTs content, the hardness of MWCNTs/Al composite increases. When the volume fraction of MWCNTs is 6%, the microhardness is HV 63.4 which is 1.9 times as hard as Al made after 5 times of FSP.